upper jaw, with one or two minute incisors on each side of it. On either side 

 of the incisors come the canines, never more than one in each quarter. These 

 are holding or stabbing teeth, long in carnivores, much reduced or lacking in 

 herbivores, and never present in rodents. Following these come the cheek 

 teeth or grinders, technically divided into premolars and molars, but the dis' 

 tinction frequently not apparent. Their number and surface are important 

 clues to the identity of their owners. In carnivores they are frequently modi' 

 fied into cutting instruments, food being bolted in chunks without chewing. 

 In grass, twig and grain eaters the enamel is often infolded to create a series 

 of ridges like those on a lower millstone. In the even-toed hoofed mammals 

 these folds usually form a crescentic pattern; in rodents they may form a 

 complicated series of loops and triangles. The tooth formula is usually given 

 for half the upper and half the lower jaw. Thus 2/2 . 1/1 . 2/2 . 3/4 would 

 mean that the animal had thirty-four teeth altogether or two incisors, one 

 canine, two premolars and three molars above on each side; and the same, but 

 for an additional molar, on each side of the lower jaw. 



SKULL CHARACTERS 



These are illustrated, and the beginner is advised to take a skull, such 

 as that of a dog, and learn the arrangement of the bones. This arrangement 

 is remarkably constant, although the relative proportions vary greatly. Special 

 notice should be given to the nasal'lacrimahmaxillary area, which varies much 

 in different members of the hoofed animals; the post-orbital process and ante' 

 orbital opening, which are of special value in keying rodent skulls; the zygO' 

 matic bar or arch below the eye region, and, on the underside, the bulla or 

 swelling below the ear region. It should be kept in mind that the orbit or eye' 

 socket is seldom partitioned off from the temporal fossa, through which the 

 muscles and the ascending process from the lower jaw pass to the side or top 

 of the cranium. The orbit, however, lies largely before the brain case and 

 is bounded posteriorly by a group of openings or foramina through which 

 nerves pass from the brain to the eye. 



OUTLINE OF CLASSIFICATION OF NATIVE MAMMALS 



Class MAMMALIA 

 Order MARSUPIALIA 



Young born at an early stage of development, usually completing 

 development in parent's abdominal pouch; epipubic bones present; 

 palate with large fenestrations; inner angle of lower jaw inflected 

 Family DIDELPHIIDAE Opossums 



With ten incisor teeth in upper jaw; eight in lower; tail 

 naked and prehensile 

 One genus — Didelphis 



473 



